SANAA, Yemen (CNN) --U.S. officials allowed a freighter with 15 Scud missiles aboard to proceed to Yemen Wednesday after the Yemeni government said it purchased the missiles from North Korea.

U.S. and Spanish forces seized the freighter So San in the Indian Ocean Monday about 600 miles south of Yemen. Yemeni officials protested the seizure, and U.S. officials released the vessel after receiving assurances that the missiles would not be transferred to a third party.

"There is no provision under international law prohibiting Yemen from accepting delivery of missiles from North Korea," White House spokesman Ari Fleischer said. "While there is authority to stop and search, in this instance there is no clear authority to seize the shipment of Scud missiles from North Korea to Yemen. Therefore, the merchant vessel is being released." (The law on sea searches)

One U.S. military official said, "The merchant vessel has now been released, yet the boarding should send a strong message to proliferators everywhere."

Yemeni officials said Wednesday they bought the missiles some time ago for their armed forces. Foreign Minister Abubakr Al-Qirbi assured the United States the missiles would not reach any third party, the Yemeni news agency SABA reported.

Yemeni officials said the So San was expected to reach Yemen's territorial waters within 24 hours. Fifteen Scud missiles were concealed beneath sacks of cement aboard the ship, which U.S. intelligence tracked from a port in North Korea.

In an interview Wednesday afternoon, Al-Qirbi said that was a North Korean decision, not his government's.

"The way they were shipped was not the responsibility of Yemen," he said. "This was the responsibility of North Korea, and you should ask them why they shipped them in this manner."

Yemeni President Ali Abdullah Saleh told U.S. Vice President Dick Cheney the missiles would be used only for the defense of his country, SABA reported.

The ship was intercepted Monday by a Spanish frigate acting on information from U.S. authorities. (View map)

In Madrid, a senior Spanish government aide said the Spanish Defense Ministry was perplexed by the decision to release the ship.

He said U.S. Secretary of State Colin Powell called Spanish Foreign Minister Ana Palacio to extend his appreciation of the maritime intercept. But, he said, in the Spanish government's view Powell couldn't offer any explanation for the decision to release the vessel. (Full story)

Yemen's foreign ministry sent a letter to U.S. Ambassador Edmund J. Hull protesting the interception of the So San by the Spanish navy, and the seizure sparked urgent consultations between U.S. and Yemeni officials.

Al-Qirbi and Fleischer both denied earlier reports that Yemeni officials did not acknowledge the shipment when first asked about it.

Earlier, U.S. officials had defended the seizure, saying it was consistent with a newly announced Bush administration policy to interdict shipments of arms capable of carrying weapons of mass destruction.

U.S. officials stressed the ship did not appear to be headed to Iraq, which used Scuds in 1991 during the Persian Gulf War. The ballistic missile can have a range of up to 400 miles, but it was unclear what type of Scud was aboard the ship.

Yemen: Missiles bought for 'defensive reasons'

A senior official told CNN the Bush administration made clear it did not believe Yemen had any strategic need for such weapons, and also vehemently protested any weapons deals involving North Korea.

But Al-Qirbi said the missiles were purchased for "defensive reasons."

A model of a Scud-B missile produced in North Korea is displayed at the War Memorial Hall in Seoul

"Nowadays, every country can consider itself to be threatened," he said. "There are world problems, there are conflicts in many parts of the world, and Yemen has to be careful and aware of all these."

Another U.S. official said the missile discovery confirms U.S. suspicions that North Korea is a significant weapons proliferator, but is not expected to alter U.S. policy toward the communist country.

"Obviously this was suspected by American authorities for some time and I think it is what it is," U.S. Deputy Secretary of State Richard Armitage said shortly after arriving in Beijing, China, for talks. (Full story)

News of the ship's interception came amid increased tension between the United States and North Korea. (Full story)

Ship had been under surveillance

Asked why the United States and others did not wait until the vessel was closer to Yemen before acting, another U.S. official said the choice was between "plausible deniability" or to "slap 'em in the face."

In other words, by intercepting the ship so far away, coalition authorities gave Yemen's government the opportunity to deny the allegations. Yemen has played a key role in the U.S.-led war on terrorism.

Yemen was not, in the opinion of the United States, "looking for missiles for terrorism," a senior administration official told CNN, adding that Yemen is "in an area of the world where respect, prestige and protection come from the barrel of a gun."

One U.S. senior official told CNN the ship appeared to be a "stateless vessel" and that there was little official paperwork onboard.

However, Trillo said it was flagged by Cambodia, though there were no identifying marks on it. The Spanish Defense Ministry considered the vessel a "pirate ship" operating illegally. (Full story)

Pentagon officials said the United States had been tracking the So San since it left North Korea several weeks ago. U.S. authorities arrived on the scene after the interception, and an explosives ordnance disposal team boarded the ship.

The authorities called in an American ordnance team after they found metal cargo containers concealed beneath sacks of cement. There was concern that the suspect containers might be booby-trapped.